1. Field of the Invention
The present invention relates to a fixing apparatus that fixes an image formed on a sheet by heat and an image-forming apparatus that uses such a fixing apparatus.
2. Description of Related Art
An image-forming apparatus of electrographic type such as a printer and copier has been known in the past. Such an image-forming apparatus has formed an image on a sheet by carrying out a series of transferring the image on the sheet and then fixing the image to the sheet. The image-forming apparatus includes a fixing apparatus that performs a fixing process. The fixing apparatus includes a fixing heater that performs any heat fixing.
It is desirable in the fixing heater that times of on/off in a hysteresis control are limited in order to reduce a temperature ripple in a fixing process. Accordingly, heater turn-on duty (hereinafter, also referred to as “duty”) while a heater turns on during each control period of time and by which power to be supplied to the fixing heater is determined has been adjusted until now by any phase control or the like. This causes power to the fixing heater to be controlled so that the fixing heater can reduce any temperature ripple. For example, Japanese Patent Application Publication No. 2000-347530 discloses a technology to adjust power to be supplied to the fixing heater by detecting any variation in commercial alternating current (AC) voltage applied to the fixing heater and adjusting the duty by the phase control. In order to reduce a generation of a flicker, the phase control is also used to prevent an inrush current from flowing when turning on the fixing heater. Further, it is preferable to cope with standards of harmonics, terminal noises other than the flicker.
Here, the flicker is referred to as such a phenomenon that a device, for example, lighting equipment that is connected to an alternating current power supply which is also connected to the image-forming apparatus flickers based on a sudden variation in the voltage generated for every time of the turn-on/off of the fixing heater. Additionally, in order to reduce the temperature ripple, switching frequency of the turn-on/off of the fixing heater increases so that the flicker has remarkable influence on the lighting equipment or the like.
When reducing power consumption of the image-forming apparatus, heat capacity of the fixing apparatus has often reduced because the power reduction of the fixing apparatus has large influence thereon. However, when reducing the heat capacity of the fixing apparatus, the temperature ripple in the fixing apparatus is remarkable so that this temperature ripple may have large influence on the fixing performance of the fixing apparatus.
It is effective to perform any power control in order to reduce the temperature ripple in the fixing apparatus. As such a power control, a control to adjust the duty by the phase control or a pulse width modulation (PWM) control has been used. It, however, is desirable that the image-forming apparatus also satisfies various kinds of standards such as disturbance voltage, harmonic distortion regulation, flicker regulation and the like. Accordingly, as one of the power controls that are effective to these standards, half cycle duty (HCD) control in which the fixing heater selectively turns on or off based on a half of the alternative current cycle as one unit has been proposed.